Method of bleaching vegetable fibrous substances



Patented Nov. 8, 1932 UNITED STATES PATENT OFFICE ERICK KAYSER, OF FRANKFORT-ON-THE-MAIN GRIESHEIM, GERMANY, ASSIGNOR TO I. G. FARBENINDUSTRIE AKTIENGESELLSGHAIT, 0F FRANKFORT-ON-THE-MAIN,

GERMANY No Drawing. Application filed November 22, 1929,

This invention relates to improvements in the process of bleaching vegetable fibrous substances, for instance in the wood cellulose bleach.

When bleaching vegetable fibrous substances, especially wood cellulose by means of hypochlorite solutions as bleaching agents, experience has shown that the bleach at first proceeds rapidly, but subsequently is steadily retarded, thebleach progressing at definite speed according to the laws of monomolecular reactions. The progress in the bleaching effect as observed, as well as the consumption of chlorine from the solution corresponds with these facts. The accumulation of incrustations originally contained in the untreated fibre and dissolved in the bleaching bath during the bleach entails, as we know, the danger of colloidal precipitations forming upon the fibers, when the treated material in a subsequent stage of the process is washed out with water during the so-called dilution wash, thus increasing the contents of ashes in the bleached material to a relatively high degree. Numerous propositions have been made to avoid this inconvenience; for instance, a method is known for carrying out the bleaching of cellulose according to the counter-current principle. According to this method the material is impregnated with the bleaching solution, then, as far as possible, freed from the partially exhausted bleaching solution in a suitable concentrator and impregnated anew with a more-concentrated bleaching solution. However, in spite of certain advantages, this process presents the drawback of calling for repeated operations and above all of prolonged duration.

Now I have found that the bleaching period can be considerably curtailed bylowering the temperatures of the individual bleaching solutlons simultaneously with increasing the concentration in such a manner that the weakest bleaching solution (or final solution) has the highest temperature of, say, about 2535 C., whereas the strongest bleaching solution (or initial solution) is applied at, say, about 1525 C. It is understood that also other ranges of temperatures may be employed. The gradation of the con- METHOD OF BLEACHING VEGETABLE FIBROUS SUBSTANCES Serial No. 409,179, and in Germany March 1, 1928.

centration of the bleaching agent in the bleaching solution should be adjusted to the bleachabilit of the material to be treated.

Thus, for dlfiicultly bleachable materials the absolute concentrations should be higher and the differences between the successive steps larger than for easily bleachable substances. I preferably subject the raw material to the bleaching treatment in the form of pasteboard sheets. This bleaching method possesses considerable advantages as the repeated concentration of the material is done away with.

Now I have found that the application of the counter-part principle in the bleach of paste-board sheets by combining it with adapting the temperature of the bleaching solution inversely to its chlorine contents, entails a very great reduction of the bleaching period and a complete utilization of the bleaching solution. For instance, a running sheet of cellulose paste-board is impregnated in three successive sections a, b, c, with bleaching solutions of different concentration. Thus the section a (comprising the raw material) receives the bleaching solution a containing a limited chlorine content, say, about 10 grams of available chlorine per liter, which is practically exhausted by interaction with the raw material. When the sheet advances to the place I), the stronger bleaching solution 1) containing about 20 grams per liter of available chlorine is brought to react, then pressed out and is further employed (its chlorine content being reduced to about 10 grams per liter) as a solution a for the next batch. Finally the sheet reaches the position 0 where the strongest bleaching solution a enters, showing an initial contents of available chlorine of about 30 grams per liter, and, after having exercised its bleaching action, is pressed out and withdrawn forming now solution 6 for the next batch. After having undergone this bleaching treatment, during which the temperature of ing been carried away by the solutions a, b, c',) is thoroughly washed only once. The final product is a completely or partly bleached material, as desired, free from incrustrations', and this product is obtained within a considerably shorter time than formerly.

The improvement in the new bleaching process for paste-board sheets thus consists in combining the counter-current principle for the raw material and the bleaching solution, and adapting the temperatures and the concentration of the bleaching solutions so as to regulate the temperature in an inverse sense to the chlorine contents in the bleach ing solution. Said improvement afiords, on the one hand, the advantage of the incrustations being completely removed from the material under treatment by squeezing off the solutions, and, on the other hand, permits of accomplishing the bleach in the shortest time.

I claim In the method of bleaching vegetable fibrous substances, especially wood cellulose, in which method the raw material is treated in form of a paste-board sheet and in countercurrent with bleaching solutions of successively increasing concentration of available chlorine, the improvement which comprises adapting the temperature of the respective bleaching solutions inversely to the chlorine content of said b eaching solutions.

In testimony whereof, I aflix my signature.

' ERICH KAYSER. 

